1. Field of the Invention
The present invention relates to high-frequency signal lines, and particularly relates to a high-frequency transmission line connected between an antenna end and a connector end.
2. Description of the Related Art
In electronic apparatuses that handle high-frequency signals, such as mobile communication terminals, a high-frequency transmission line for transmitting high-frequency signals is used in a signal processor. For example, in mobile communication terminals, a coaxial cable of 50Ω or 75Ω is used.
A connector may be provided between such a coaxial cable and a high-frequency signal processor, as disclosed in, for example, Japanese Unexamined Patent Application Publications No. 2003-060425 and No. 2004-064282. FIGS. 1A to 1C illustrate an example thereof. FIG. 1A is a cross-sectional view of a coaxial cable 100, and FIG. 1B illustrates a state where a connector 40 is attached to one end of the coaxial cable 100.
For example, in a case where an antenna is connected to a first end of a high-frequency transmission line such as a coaxial cable, and a connector is connected to a second end of the high-frequency transmission line, a high-frequency signal received by the antenna is transmitted to a high-frequency signal processor via the coaxial cable and the connector.
In ordinary cases, however, the characteristic impedance of the antenna is lower than the characteristic impedance of the coaxial cable (normally 50Ω or 75Ω), whereas the characteristic impedance of the connector is higher than the characteristic impedance of the coaxial cable. Accordingly, resonance occurs at a frequency at which a standing wave of a quarter wavelength multiplied by an odd number develops in the coaxial cable.
FIG. 1C is a diagram illustrating that state. In FIG. 1C, in a case where an antenna is connected to a first end FP, and a connector is connected to a second end SP, because the impedance is low at the first end FP and the impedance is high at the second end SP, resonance occurs at a frequency at which a standing wave develops in which the first end FP is a minimum voltage point (short-circuit end) and the second end SP is a maximum voltage point (open end).
Here, one wavelength in the coaxial cable 100 is represented by λg, the length of the coaxial cable 100 is represented by Lg, and the relative dielectric constant of the dielectric material of the coaxial cable 100 is represented by εr. In this case, a resonance frequency fo of a fundamental wave at which quarter-wavelength resonance occurs is expressed by the following equation (1).fo=1/(4Lg√εr)×c (c: velocity of light)  (1)
In a case where Lg=9 cm and √εr=1, resonance in a basic mode occurs at about 830 MHz. Thus, the cutoff frequency of the coaxial cable 100 is lower than about 830 MHz. In this case, for example, in the case of transmitting a signal in a 900 MHz band, an insertion loss in the coaxial cable 100 is a problem.